Semiconductor devices are very small and delicate, and must be protected from physical and environmental damage. Traditionally, the devices have been enclosed in a metal can, encapsulated by covering or potting with an organic material, or encapsulated in plastic by casting or molding. The latter method, known as transfer molding or overmolding, places the semiconductor device, which has been electrically connected to a lead structure, lead frame, or circuit carrying substrate, in a mold cavity. A thermoset material is molded around the semiconductor device to form a solid monolithic unit that is sealed from environmental damage and is rugged enough to withstand physical damage while being assembled onto a circuit board. This technology may be used to form a package for an individual component, or may be used to encapsulate a semiconductor device that has been mounted directly on a circuit carrying substrate.
The molding resin is preformed into a mass of precisely calculated size and shape, and placed into a hydraulically operated transfer mold. A plunger forces the resin through the runners and gates, into the cavity portion of the mold. The molding resin is injected into the individual mold cavities under pressure to flow around the semiconductor devices and form the molded package. During injection and flow, the molding material begins to cure and continues curing for a predetermined time after flow has stopped. After the specified curing time, the mold is opened and the molded packages are taken out of the mold tool and separated from the runners and gates. The amount of molding material is critical, in that there must be adequate material to completely fill the mold cavity so that no voids are left in the molded semiconductor packages. Excess molding material forces the mold to open and creates flash around the edges of the molded packages, necessitating undesirable touch up operations.
Molding resins used for semiconductor encapsulation are classified as thermoplastic or thermoset, with thermoset being the predominant type of resin. Thermoplastics typically exhibit problems such as melting or blistering during soldering of the semiconductor package, and the high moisture uptake of these resins leads to loss of dielectric properties. Silicone, epoxy and silicone modified epoxy resins are the thermoset resins used for encapsulating semiconductor devices. However, silicones and silicone-epoxy resins have essentially been replaced because of the lower price and higher reliability of modern epoxy resins. Modern electronics applications require a high heat resistance and high moisture resistance that epoxy-novolac resins can offer. The introduction of low pressure transfer molding techniques has resulted in the broad use of novolac-modified epoxy resins.
When used to encapsulate a semiconductor device that has been mounted directly on a modified epoxy resin substrate, the epoxy-novolac resins suffer from poor adhesion to the substrate. Overmolded devices rely entirely upon adhesion of the molding compound in order to bond the overmold compound to the substrate, and are prone to adhesion failures at this interface. Minor amounts of contaminants in the molding compound or on the surface of the substrate can cause adhesion failure, as will flexing or other mechanical stress of the substrate. Adhesion failure can result in corrosion, or catastrophic failure of the device by damaging the delicate electrical connections or cracking of the semiconductor material. Clearly, an improved method of bonding the overmolded compound to the substrate is needed.